1. Field of the Invention
The invention relates to compensation for transmitter instability, generally, and more particularly to pulse microwave transmitters used in MTI systems.
2. Description of the Prior Art
The general subject of moving target indication radar (MTI) is extensively documented in the patent and other technical literature. Chapter 17 of the book "Radar Handbook" by Merrill Skolnik (McGraw-Hill 1970) provides a general background, with bibliography, for a general understanding of the state of this art.
It is generally understood that instabilities including phase and frequency jitter, amplitude-jitter, and pulse-width and timing-jitter, impose limitations on achievable MTI performance due to the separable source of instability set forth in Table 1, taken from the aforementioned "Radar Handbook", as follows:
TABLE 1 ______________________________________ INSTABILITY LIMITATIONS ______________________________________ Pulse - pulse instability Limit on improvement factor Transmitter frequency shift ##STR1## Transmitter phase shift ##STR2## Transmitter amplitude jitter ##STR3## Transmitter timing jitter ##STR4## Transmitter pulse width ##STR5## where .DELTA. f = interpulse frequency change .tau. = transmitted pulse length .DELTA. .phi. = interpulse phase change A = pulse amplitude, volts .DELTA. A = interpulse amplitude change .DELTA. t = time jitter B .tau. = time-bandwidth product (B .tau. = 1 for uncoded pulses) .DELTA. PW = pulse-width jitter ______________________________________
In a magnetron transmitter for MTI, the most common technique used to achieve phase stability from pulse-to-pulse is the "locked-coho". An example of this technique, as usually implemented, is shown and will be hereinafter briefly described in connection with FIG. 1.
Another potentially useful stabilization approach is offered by "priming" the magnetron with a coherent low-power oscillator. Priming input power of -30 dB with respect to the magnetron output power has been found to be relatively effective in many cases.
Transmitter stabilization may also be achieved by using the "feed-forward" technique wherein the signal to be stabilized is delayed with respect to the error signal (the opposite of the "feed-back" technique). Feed forward stabilization is not applicable to a single power oscillator such as a magnetron, but is of potential value in those systems which are completely coherent, i.e., those systems which are composed of a single low level oscillator followed by a chain of power amplifiers. Feed forward stabilization may be of value for those systems using the primed magnetron principle if the priming source is comparatively unstable.
None of the above methods is completely satisfactory however, in that they all require large additional amounts of analog hardware and restrict system dynamic range. The manner in which the device of the present invention deals with the aforementioned problems will be understood as this description proceeds.